91色情片

IT鈥橲 hard to believe we could fail to recognise people with whom we鈥檝e been
intimate. Yet geneticists in the US have created socially inept rodents with
just that failing. They provide the clearest picture yet of how a molecule
popularly known as the 鈥渓ove hormone鈥� shapes relationships in mammals.

In animals as different as mice, voles, monkeys and humans, the hormone
oxytocin is released from the brain when relationships are being forged, such as
during mating or maternal nurturing. But just what oxytocin does has been
difficult to work out. When injected into an animal, for example, the hormone
can impair or promote bonding, depending on how it is given.

Now James Winslow of Emory University in Atlanta and his colleagues have
provided strong evidence that oxytocin promotes 鈥渟ocial memory鈥濃�攖he
development of familiarity between individuals.

When male mice encounter a female, they routinely sniff the newcomer to
determine whether she is an appropriate mate. If normal males are repeatedly
exposed to the same female at intervals of a minute, this period of exploration
shortens with each encounter, from 40 seconds to just 10 seconds on the fourth
meeting.

But males genetically engineered to lack oxytocin behave quite differently.
The female gets the full 40-second scrutiny on each encounter, says Winslow.
鈥淓ach time, he acts like he has never seen her before.鈥�

The defect seems to be restricted to the social realm. The mice without
oxytocin react normally to other smell cues and can learn to navigate mazes,
showing their perception and general memory are intact. And a dose of oxytocin
quickly cures them of their indifference to old acquaintances.

Geert De Vries, a neuroscientist at the University of Massachusetts, Amherst,
says the results are very convincing. 鈥淲e know very little about how social
recognition works in any species,鈥� he says. He believes the study will stimulate
new thinking about the issue.